Manufacturing of many types of discrete products can benefit from multi-variable computerized controllers. The semiconductor wafer processing area is an example of an area of particular interest. The small feature sizes and the large number of steps required to fabricate state of the art integrated circuits (on semiconductor wafers) make it essential that each of the process steps meet a tight set of specifications. Since process variations are inevitable, statistical process control (SPC) is one of the techniques used for controlling fabrication processes. Often, implementing SPC on a wafer-by-wafer basis requires having equipment with in-situ sensors for measuring the desired quality characteristics, or making changes to existing equipment to incorporate in-situ sensors. Obtaining new equipment with in-situ sensors or modifying existing equipment can be expensive. In addition to the cost of the sensors, adding sensors to existing equipment can cause expensive downtime and decrease mean time between failure (MTBF).